This invention relates to metal and conductive mineral or electrically conductive object detectors in general and to inductive metal detectors in particular of the type having a transmission coil for radiating frequencies of alternating electrical signals and a reception coil for receiving the same.
Numerous metal detectors of the general type contemplated in the present invention have been developed and utilized in the past. These are comparatively expensive and complex devices which have required a relatively high degree of training and skill for effective utilization. The transmit/receive metal detectors which the prior art generally constitutes have not been easily usable in highly mineralized soil since the detectors became inoperative or difficult to use in that the pick-up coil, when it was placed in close proximity to the ground, would produce no usable signal. If the detector was retuned, a capability present in some of the prior art, then when the detector head was close to the ground a signal from desired metal objects could be detected but if the head was allowed to move up or down in relationship to the ground, a signal would be produced that would fluctuate with the head motion which made the use of such detectors very difficult and unsatisfactory.
The cause of this difficulty is that when magnetite and other conductive minerals in the soil enter the radiated electromagnetic field from the transmit coil two things happen. First, the amplitude of the received signal in the received coil changes because of mutual coupling between the coil and the conductive minerals. Secondly, the phase of the signal in the received coil changes. The resulting voltage in the received circuit will be affected by both amplitude and phase changes and the net voltage will either be greater or lesser than that originally present due to the algebraic addition or subtraction of the two components of change.
The prior art devices of the type known lack tuneable ground rejection circuit means which can be adjusted to provide the desired degree of ground coupling discrimination. Some ground discrimination circuits have been utilized in the past, but these have generally been of the sort in which the audio or other user signal is turned down to a less objectional level. Alternatively, the power supply voltage operating the transmit coil was turned down to decrease the received signal level due to ground coupling. Such circuits are delicate and require continual adjustment in the field if any usable searching for hidden metallic objects is to be achieved.
A related discrimination capability found in the prior art deals with what is called "junk" discrimination. This type of discrimination refers to a nullification of signals coming from unwanted types of small metal objects. These objects can, due to electrical and inductive characteristics, be selectively excluded by returning the transmit and receive coils in a manner which eliminates signals being produced of sufficient magnitude in the receive coil to alert the operator. By a selective feedback circuit from the transmit coil to the receive coil, some prior art devices have produced a desirable so called, "junk rejection" capability. This excludes signals from unwanted objects within the given range of size and electrical characteristics. Such prior art devices have, however, lacked tuneable ground rejection and have been extremely difficult to use, especially when the desirable metal objects may be found in mineralized soils.